1. Field of the Invention
This invention relates to recovery of olefins from pyrolyzed hydrocarbon gases and/or refinery off-gases and especially relates to recovery of olefins by absorption with a preferential physical solvent from de-acidified, compressed, and dried hydrocarbon gases containing olefins.
2. Review of the Prior Art
Olefins such as ethylene and propylene are present in thermally or catalytically cracked gas streams or in refinery off-gases and are commonly associated with large quantities of hydrogen. These gases generally comprise methane, carbon monoxide, carbon dioxide, acetylene, ethane, methyl acetylene, propadiene, propylene, propane, butadienes, butenes, butanes, C.sub.5 's, C.sub.6 -C.sub.8 non-aromatics, benzene, toluene, xylenes, ethyl benzene, styrene, C.sub.9 -400.degree. F. gasoline, 400+.degree. F. fuel oil and water.
Numerous processes are known in the solvent absorption art for isolation and recovery of olefins from cracked, refinery, and synthetic gases containing these unsaturated compounds. Some processes utilize specific paraffinic compounds as an absorption oil, and others utilize an aromatic absorption oil as a solvent within an absorber column or an absorber-stripper column having a reboiler.
Thermal cracking of hydrocarbon feedstocks in pyrolysis furnaces for production of ethylene has been an established technology since the 1940's. The pyrolysis furnace gases were sent to the recovery section of an ethylene plant in which the first fractionation column was a front-end demethanizer operating at about -150.degree. C. The deacidified, compressed, dried, and chilled pyrolysis gases were fed to the demethanizer after five compression stages to 500 psia. The demethanizer bottoms were fed to a deethanizer, and the demethanizer overhead, rich in hydrogen, was fed to a cryogenic unit which recovered additional ethylene from the fuel gas stream. A back-end acetylene removal system, such as a series of two acetylene reactors, was typically located between the deethanizer and the C2 splitter or between the depropanizer and the C3 splitter. This arrangement caused the production of large amounts of green oil, a polymer formed from olefins and diolefins, which was likely to freeze in the C2 splitter or accumulate in the ethane vaporizer.
In 1971, G. M. Clancy and R. W. Townsend proposed a heat pumped depropanizer in "Ethylene Plant Fractionation", Chemical Engineering Progress, Vol. 67, No. 2, Pages 41-44, as the front end distillation column for receiving the compressed gas from the fourth compression stage after passage through a single catalytic acetylene hydrogenation reactor having no need for regeneration because of the high partial pressure of hydrogen in the gases being treated. This reactor sequence produced no green oil, provided stable flow rates at all plant throughputs, eliminated fouling in the high-pressure stage of the cracked gas compressor, eliminated polymerization of butadiene in the deethanizer and its reboiler, enabled ethylene and propylene purity specifications to be met more easily, simplified operation and maintenance, and reduced capital, horsepower, and operating costs for the plant.
W. K. Lam and L. Lloyd discussed a theory for selective acetylene hydrogenation in "Catalyst Aids Selective Hydrogenation of Acetylene", The Oil and Gas Journal, Mar. 27, 1972, pages 66-70. They explained that the catalyst contains 0.04% palladium impregnated on its alpha-alumina support which is calcined so that the surface area and pore volume lie within carefully controlled limits.
W. K. Lam and A. J. Weisenfelder discussed capital and operating costs in "Low-Capital Ethylene Plants", as presented at the AIChE Spring National Meeting, Apr. 6-10, 1986 in New Orleans, La., for a fractionation sequence of deethanizer - demethanizer - C2 splitter, in which the gross deethanizer overhead, containing the C2 and lighter fraction of the cracked gas, was compressed in the last stage of the cracked -gas compressor and then fed to a selective, front-end, catalytic hydrogenation reactor.
In a paper presented at the Petrochemicals Session at PACHEC '88, Oct. 19-21, 1988, entitled "Theory and Reaction Mechanism for Commercial Selective Catalytic Hydrogenation Reactors", W. K. Lam discussed acetylene removal processes in ethylene plants, characterizing them as divided into four types:
1. solvent absorption of acetylene, which was used in the early 1950's; PA0 2. cracked gas train hydrogenation reactors, available in the late 1950's; PA0 3. back-end catalytic hydrogenation reactors, available in the early 1960's and now commonly used; and PA0 4. front-end selective catalytic hydrogenation reactors. PA0 a) the liquid-phase primary cracked gas drier; PA0 b) regeneration facilities including a furnace for the back-end acetylene reactors; PA0 c) green oil removal facilities; PA0 d) the propylene product drier; and PA0 e) the high pressure condensate stripper.
Lam analyzed reaction rates and postulated a theory for selective hydrogenation of acetylene involving the displacement and/or exclusion of ethylene from active sites of the palladium catalyst by selectivity moderators, such as propadiene, methyl acetylene, and carbon monoxide, in order of increasing reactivity.
At the AIChE Ethylene Producers' Conference, March 19, 1990, W. K. Lam, S. C. K. Cua, and K. F. McNulty discussed reaction theory in terms of a kinetic and dynamic model for the front-end acetylene hydrogenation reactor that accurately predicted its operation during changing operating conditions.
U.S. Pat. No. 3,691,251 proposed the use of a lower cost desiccant, such as an activated alumina, for the top two-three feet of the molecular sieve bed in a downflow drying operation for a cracked propane stream containing ethylene and other unsaturated constituents, e.g., dienes which deposit or form polymers or otherwise plug a desiccant, such as a molecular sieve, causing maldistribution of the cracked gases and inadequate drying.
In U.S. Pat. No. 4,345,105, methyl acetylene and propadiene are removed from a stream by hydrogenation in order to minimize any danger of violent decomposition or explosion or coke formation.
U.S. Pat. No. 4,540,422 points out that in the fractionation to separate propylene from propane in a stream that has been recovered from de-ethanized and de-butanized gas cracking product, the concentration of methylacetylene and propadiene in the bottoms from the fractionation increases proportionally as the concentration of propane in the feedstock decreases. Particularly when the propane stream is recycled to the gas cracking operation, the high content of acetylenes is potentially explosively hazardous.
This ethylene recovery process, utilizing a heat pumped depropanizer in combination with a front-end selective catalytic hydrogenation reactor for acetylene removal, has been a very useful advance in the art, but it is nevertheless characterized by high energy costs so that there is a real need for modifications that conserve energy.
In U.S. Pat. No. 4,743,282, Y. R. Mehra disclosed the replacement of the low-temperature fractionation train of an olefin producing facility with an extractive stripping column employing a preferential physical solvent which is selective for ethylene and heavier hydrocarbons. Simulated performance showed that solvent losses and product purity were significantly better than prior art absorption processes of ethylene recovery.
In U.S. Pat. No. 4,832,718, Y. R. Mehra disclosed a process for contacting an olefins-containing gas stream at no more than 500 psia with regenerated solvent to produce an off-gas stream of hydrogen and methane and an ethylene-plus product stream while avoiding operation near the system critical pressure as evidenced by the difference between liquid and vapor density being less than 20 pounds per cubic foot. Paraffinic and naphthenic solvents of a specified range of molecular weights and UOP characterization factors, in addition to benzene and toluene, were disclosed as satisfactory solvents.
In U.S. Pat. No. 5,019,143, Y. R. Mehra described a continuous process for contacting an olefins-containing feed gas stream in a demethanizing-absorber column, having at least one reboiler, with a specified lean physical solvent stream to produce a rich solvent bottoms stream containing ethylene and heavier hydrocarbons and an overhead stream containing the remaining lighter components of the feed gas, then regenerating the rich solvent stream in a distillation column, having at least one reflux condenser and at least one reboiler, to produce the ethylene plus hydrocarbon product as an overhead stream, without further need for demethanizing the ethylene plus product by cryogenic fractionation, and the lean physical solvent as a bottoms stream for recycling to the contacting step. This process, illustrated in FIGS. 8 and 9, was suitable for all hydrocarbon gas streams containing at least 5 mol % of ethylene. In the cryogenic demethanization sequence, after feedstock cracking and water washing, the cracked gas is compressed, dried, and subcooled to -150.degree. F. (-101.degree. C.) and lower to condense out hydrocarbons prior to demethanization. Because methane is a light gas and has a very low boiling point, it must be distilled under pressure and condensed at about -142.degree. F. (-97.degree. C.) with ethylene refrigerant. At this and other low temperatures in the process, ordinary carbon steel becomes quite brittle. More expensive nickel-bearing steel must be used to fabricate the distillation column and associated equipment.
An important factor for ethylene plant design during the 1990's is that most of the recent pyrolysis furnaces being built or commissioned are of the very low residence time design (0.1 to 0.2 second) which produces high yields of ethylene but increases acetylene production two to three times over that of the higher residence time crackers of the early 1980's, so that back-end acetylene reactor systems have had many operational problems, such as high temperature rise across the back-end reactor, hydrogenation of large amounts of ethylene due to non-selectivity, and high production of green oil. Another factor is that the specification of acetylene in ethylene product is now commonly set at 1 ppmv or less.
In contrast, the front-end heat-pump deethanizer and depropanizer process sequences have many advantages, especially when used with a front-end selective catalytic hydrogenation acetylene reactor system. Such a front-end reactor provides cooler effluent because the gases are greatly diluted by the presence of hydrogen and methane. The front-end reactor also enables the hydrogen in the process stream to be used for hydrogenation, minimizes catalyst fouling so that frequent on-site catalyst regeneration is not required, eliminates green oil production, and provides ethylene and propylene gain across the reactor so that production from the plant is significantly increased because acetylene is selectively hydrogenated to ethylene and around 80% of the methyl acetylene (MA) and 20% of the propadiene (PD) are selectively converted to propylene. The front end hydrogenation step consequently reduces the amount of methyl acetylene and propadiene to be hydrogenated in the tail-end MAPD reactor. In addition, the combination of the front-end reactor and the depropanizer or deethanizer as the front-end column provides greater stability and flexibility for the operation of an ethylene plant, so that it may be employed over a range of feedstocks from ethane and propane to atmospheric gas oil, and the system is less subject to disturbances due to turndown or composition changes resulting from the cyclical operation of the pyrolysis furnaces. Certain process operations and/or equipment items normally required in a conventional front-end demethanizer ethylene plant are also eliminated, comprising:
It is characteristic of all conventional ethylene recovery plants, whether the front-end column provides demethanization, deethanization, or depropanization, that a refrigeration system is required for separation of methane and ethylene. The required cryogenic temperatures for such a refrigeration system necessitate use of 31/2% nickel in all drums and heat exchangers and stainless steel in all piping and thereby increase the total plant installed cost.
By utilizing an absorber-stripper column to treat the vapor from the condenser of the depropanizer column, Mehra has shown that the following process operations and/or equipment items can be eliminated, thereby saving significant capital and operating costs for the plant:
a) the ethylene compressor, the ethylene condenser, and cascading operation of the ethylene-propylene refrigeration system, thereby greatly reducing maintenance costs and contributing significantly to the ease of start-up and on-going operations;
b) the high-pressure stripper in the compression train;
c) the conventional low temperature demethanizer feed chilling train, enabling replacement of the multitude of cold exchangers together with their associated low temperature piping by fewer exchangers using killed carbon steel; and
d) the methane compressor, if low-pressure demethanization is involved in conventional front-end demethanizer design.
However, it has been realized that the presence of acetylenes, diolefins, and butadienes, in particular, presents a potential for fouling equipment associated with the hot portions of the solvent regeneration system. Consequently, there exists an immediate need to provide means to significantly reduce the concentration of acetylenes and diolefins in the feed to the absorber-stripper configuration proposed by Mehra. For the absorption-based Mehra system, there also continues to be a real need to cost effectively reduce the solvent losses. Moreover, the Mehra process has been troubled by the typical asymptotic relationship of solvent circulation rates to completeness of ethylene recovery, so that there exists an additional need for a more energy effective method for ethylene recovery.